Download T T t t

Ch. 11
Important Vocab
Genetics: the study of heredity
 Fertilization: the joining of sperm and egg
 Genes: chemical factors that determine
traits (a specific characteristic)
 Alleles: different forms of a gene

 Ex: tall vs short, yellow vs green

Trait: a specific characteristic (color, size,
shape, etc.)
More Important Vocab

True-breeding: individuals that will
produce offspring identical to
themselves if allowed to self-pollinate
 TT, tt, RR, rr

Hybrid: individuals that will not produce
offspring identical to themselves if
allowed to self-pollinate
 Tt, Rr, Ss
Principle of Dominance

States that some alleles are dominant
and others are recessive
 An organism that has a dominant allele for a
particular trait will always show that trait
 An organism with a recessive allele for a trait
will exhibit that form only when the dominant
allele is absent
Who is the Father of Genetics?
Gregor Mendel
 What organism did he study?

 Pea plants

Started with true-breeding plants
 He crossed a true-breeding tall plant (TT)
with a true-breeding short plant (tt)
 Found that all offspring were tall
 TT x tt = all tall
 Why?
○ Principle of dominance
Probability and Genetics
Probability: the likelihood that a particular
event will occur
 Why do we look at probability when
studying genetics?

 All the traits you receive from your parents
depends on chance!

What do we use to predict the outcome of
a genetic cross between two individuals?
 Punnett Squares
Probabilities Predict Averages
Probabilities predict the average
outcome of a large number of individuals
 You are more likely to get the predicted
outcome with 500 individuals rather than
5 individuals.

(One factor crosses)
Vocab for Punnett Squares

Homozygous: organisms that have two
identical alleles for a particular trait
 Aka – true-breeding

Heterozygous: organisms that have two
different alleles for a particular trait
 Aka – hybrid

Phenotype: physical characteristics
 Yellow, green, tall, short

Genotype: genetic makeup
 YY, Yy, yy, TT, Tt, tt
Let’s Practice the Vocab!

T= tall, t = short
 Homozygous tall = TT
 Heterozygous = Tt
 Homozygous short = tt

Y = yellow, y = green
 Homozygous yellow = YY
 Heterozygous = Yy
 Homozygous green = yy

Let’s try some monohybrid (single
factor) crosses!
Practice with Punnetts!

Heterozygous tall x heterozygous tall
 What are the genotypes?
○ Tt x Tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
T
○ 3:1
 Genotypic ratio?
t
○ TT:Tt:tt
○ 1:2:1
T
t
TT
Tt
Tt
tt
More Practice with Punnetts!

Homozygous tall x homozygous short
 What are the genotypes?
○ TT x tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
t
○ 4:0
 Genotypic ratio?
t
○ TT:Tt:tt
○ 0:4:0
T
T
Tt
Tt
Tt
Tt
More Practice with Punnetts!

Heterozygous tall x short
 What are the genotypes?
○ Tt x tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
t
○ 2:2
 Genotypic ratio?
t
○ TT:Tt:tt
○ 0:2:2
T
t
Tt
tt
Tt
tt
Your turn to try!

Homozygous tall x heterozygous tall
 What are the genotypes?
○ TT x Tt
 Punnett Square
 Phenotypic ratio?
○ Tall:short
T
○ 4:0
 Genotypic ratio?
t
○ TT:Tt:tt
○ 2:2:0
T
T
TT
TT
Tt
Tt
(Two Factor crosses)
Back to Mendel!

When looking at a single trait (i.e. height),
the two alleles will separate from each
other
 The 2 T’s separate from each other
Now let’s look at 2 traits at the same time.
 Mendel asked:

 Does the segregation of one pair of alleles affect
the segregation of another pair of alleles?
○ Tall vs short and yellow vs green
○ No, they will separate independently. What the
alleles for height do will not affect what the color
alleles do.
Principle of Independent
Assortment
States that genes for different traits can
segregate independently during the
formation of gametes.
 Gametes: sex cells
 What are the two types of gametes?

 Sperm
 Egg
Dihybrid Crosses

Let’s look at two factors: seed shape
and color
 Round (R) vs wrinkled (r) seed shape
 Yellow (Y) vs green (y) seed color
Homozygous round and yellow = RRYY
 Wrinkled and green = rryy
 Heterozygous for both traits = RrYy

Dihybrid Crosses
Let’s cross 2 truebreeding plants:
Round yellow x wrinkled green
 Step 1: What are the genotypes?

 RRYY
 rryy

Step 2: What are the
possible gamete
combinations?
RRYY
rryy
RY
RY
RY
RY
ry
ry
ry
ry
Dyhybrid Crosses

Step 3: Draw out the Punnett Square
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Dyhybrid Crosses

Step 4: Figure out the Phenotypic ratio
 Round yellow: round green: wrinkled yellow:
wrinkled green
 16:0:0:0
Now you try!

Cross two individuals that are
heterozygous for both traits.
 Round vs wrinkled
 Yellow vs green

What are the genotypes?
 RrYy
 RrYy

What are the gametes?
 RrYy = RY, Ry, rY, ry
 RrYy = RY, Ry, rY, ry
RrYy x RrYy

RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Phenotypic ratio:
 Round/yellow : round/green : wrinkled/yellow : wrinkled green
9:
3:
3:
1
Incomplete Dominance
Codominance
Incomplete Dominance
Case in which one allele is not completely
dominant over another
 Heterozygote phenotype is somewhere
between the two homozygote phenotypes
 Example: Snapdragons

 Red (R) and white (r)
 RR = red and rr = white
 What about Rr?
○ These are pink!
Let’s Practice!
Red snapdragon x white snapdragon
 Genotypes?
R
R

 RR x rr
Punnett Square
 Ratios:

 Phenotypic
○ Red: pink: white
○ 0:4:0
 Genotypic
○ RR: Rr: rr
○ 0:4:0
r
Rr
Rr
r
Rr
Rr
Now It’s Your Turn:
Pink x Pink
 Genotypes?

R
 Rr x Rr
Punnett Square
 Ratios:

 Phenotypic
○ Red: pink: white
○ 1:2:1
 Genotypic
○ RR: Rr: rr
○ 1:2:1
r
R
RR Rr
r
Rr
rr
Codominance
Situation in which both alleles contribute
to the phenotype
 Heterozygote is usually spotted or
speckled compared to the solid colored
homozygotes
 Example: Chickens





B = black, W = White
BB = black chicken
WW = white chicken
BW = black and white speckled chicken
Let’s Practice!
Black chicken x White Chicken
 Genotypes?

B
 BB x WW
Punnett Square
 Ratios:

 Phenotypic
○ Black: speckled: white
○ 0:4:0
 Genotypic
○ BB: BW: WW
○ 0:4:0
B
W
BW BW
W
BW BW
Your Turn!
Speckled chicken x speckled chicken
 Genotypes?

B
 BW x BW
Punnett Square
 Ratios:

 Phenotypic
○ Black: speckled: white
○ 1:2:1
 Genotypic
○ BB: BW: WW
○ 1:2:1
W
B
BB BW
W
BW WW
Bloodtyping is Codominant!

Types A and B are both dominant to O
Bloodtype
A
B
AB
O
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
Antigens
A
B
A and B
None
Antibodies
B
A
None
A and B
Will take
blood from:
A or O
B or O
AB, A, B, O
O
Can’t take
blood from:
B or AB
A or AB
---
A, B, AB
Antigen: substance that triggers an immune response
Antibody: protein that helps destroy pathogens
Chromosomes
Genes are located on chromosomes
which are found in the nucleus.
 Mendel’s principles of genetics requires
2 things:

 Each organism must inherit a single copy of
every gene from each parent.
 When an organism produces its own
gametes, those two sets (copies) of genes
must be separated from each other so that
each gamete contains just one set of genes.
Chromosomes
Homologous chromosomes: term used to refer
to chromosomes that each have a
corresponding chromosome from the parent of
the opposite sex
 A cell that contains both sets of homologous
chromosomes is said to be diploid (meaning 2
sets)

 Having 2 complete sets of chromosomes agrees with
Mendel’s idea that the adult organism contains 2
copies of each gene

A cell that contains only 1 set of chromosomes
is called haploid – i.e. gametes
Chromosomes

Humans:
 How many chromosomes does each of your
typical body cells have?
○ 46 (diploid)
 How many chromosomes does each of your
gametes have?
○ 23 (haploid – one set of chromosomes)
Meiosis
Meiosis is a process of reduction
division in which the number of
chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell.
 Divided into 2 major steps:

 Meiosis I
 Meiosis II
Meiosis I

Prophase I
 Each chromosome pairs with its
corresponding homologous chromosome to
form a tetrad (made of 4 chromatids)
 Homologous chromosomes then exchange
portions of there chromatids during crossingover
○ Results in the exchange of alleles between
homologous chromosomes and produces new
combinations of alleles
Meiosis I

Metaphase I
 Spindle fibers attach to the chromosomes
 Chromosomes line up across middle of cell
at metaphase plate

Anaphase I
 Fibers pull homologous chromosomes
towards opposite ends of cell

Telophase I and Cytokinesis
 Nuclear membranes reform
 Cell separates into 2 haploid daughter cells
Meiosis II

Prophase II
 The 2 cells created by Meiosis I now enter
into a second meiotic division
 At this point, each cell’s chromosomes
contains 2 sister chromatids (same genes,
but different alleles  one may have “A”
while the other may have “a”)

Metaphase II
 Chromosomes line up at metaphase plate
 Spindle attaches to centromeres
Meiosis II

Anaphase II
 Sister chromatids separate and move
towards opposite ends of cell

Telophase II and Cytokinesis
 Nuclear membrane reforms
 Cytoplasm divides
 Result: 4 genetically different haploid
gametes
Mitosis vs Meiosis
 Mitosis
produces 2 identical
diploid daughter cells
 Meiosis produces 4
genetically different haploid
gametes
Sex-linked Traits
Human Heredity

Karyotype: a picture of chromosomes
arranged in homologous pairs
Karyotypes

Human karyotypes show 46 chromosomes
 23 chromosomes came from the sperm (father)
 23 came from the egg (mother)

Chromosome pairs #1-22 are called
autosomes.
 These determine all sorts of traits – eye color,
hair color, height, shapes of features
Sex Chromosomes

Chromosome pair #23 are called sex
chromosomes because they determine
a person’s sex
 Females have 2 X’s = XX
 Males have 1 X and 1 Y = XY
All human egg cells carry 1 X.
 Only ½ of human sperm cells carry an
X, whereas the other ½ carry a Y

 Results in a 50/50 chance of having either a
girl or boy.
 Who determines the sex of the baby? Why?
Sex-linked Genes
Sex-linked genes are genes located on the X
chromosome.
 Most sex-linked traits are recessive.
 Males are more likely to show sex-linked traits.

 This is because males have only 1 X – if this X is
“bad”, the sex-linked trait will be expressed.
 Females have 2 Xs – both Xs have to be “bad” in
order for the trait to be expressed.

Examples: Colorblindness, Hemophilia,
muscular dystrophy
Sex-linked Traits
In order to figure out the chance of
getting a sex-linked trait, we follow the
same procedure as we did for singlefactor traits.
 However, we have to add sex
chromosomes (X and Y) into the
equation.
 Let’s use colorblindness as an example.

Colorblindness - Genotypes

Males have 2 possible genotypes:
 Normal Male = XY
 Colorblind male = XcY

Females have 3 possible genotypes:
 Normal female (not a carrier) = XX
 Normal female (carrier) = XcX
○ This means she doesn’t show the trait, but
can pass it on to her offspring
 Colorblind female = XcXc
Colorblindness with Punnetts

Normal male x colorblind female
 What are the genotypes?
○ XY x XcXc
 Punnett Square
 What is the chance they will
have a colorblind son?
X
Y
Xc XcX XcY
○ 50%
 What is the chance they will
have a colorblind daughter?
○ 0%
Xc XcX
XcY